Cutting of metals such as steel by plasma gas jet or by laser, and also by water jet, is well established. Where reference is made to steel it will be understood to encompass any metals which can be jet cut by such techniques. The term jet cutting is used generically to encompass high temperature/pressure metal cutting techniques such as by plasma gas jet, or by laser cutting techniques, or water jet and the like. The term jet is used to encompass either the plasma gas jet, or a laser beam, or other cutting medium.
Jet cutting is used for making linear cuts. It is also used for making openings which may be circular and have cylindrical walls, or elongated slots with semi circular end walls. These techniques are fast and can be accurately controlled, and are generally speaking, superior to drilling such openings or cutting mechanically. However, it is known that when jet cutting openings particularly for fastenings such as bolts, through a steel work piece, the opening will preferably be circular, and must have a cylindrical wall. Even where openings are cut, which have partially arcuate surfaces, similar to slots with semi circular ends, the arcuate surfaces must be regular and even from one end to the other of the opening.
It is found that when jet cutting metal such as steel of a considerable thickness or gauge, especially when the openings are circular, they tend to be formed in a slightly frusto-conical manner. In other words they are non cylindrical. This is found to be unsatisfactory when passing fastening such as bolts through such jet cut openings.
For this reason, the drilling of such openings by mechanical drills is often preferred even though it is slower and more costly.
Another problem with jet cutting of circular or arcuate openings has been that the opening is often found to have an undesirable recess or depression in the wall. This results in the wall around the opening being irregular.
In the case of plasma cutting an inert gas is blown at high speed out of a nozzle; at the same time an electrical arc is formed through that gas from the nozzle to the surface being cut, turning some of that gas to plasma. The plasma is sufficiently hot to melt the metal being cut and moves sufficiently fast to blow molten metal away, leaving a tapered channel in the workpiece known as a cut kerf.
Relative motion between the plasma torch and the workpiece allows the process to be used to effectively cut the workpiece. The plasma gas also known as the cutting gas interacts with a shield gas to cause the plasma arc to be constricted and enabling the temperature of the torch to be lowered. The two gases then flow downstream from the nozzle orifice enabling heat and mass transfer
When cutting with plasma arc, the gas mixture ratio of the plasma gas and shield gas is set for proper operation and optimizing cut quality. For example, when cutting mild steel oxygen is used as the plasma gas and air as the shield gas. The plasma gas is ionized in the plasma process and exits through the nozzle orifice. The shield gas is the secondary gas in the plasma process. It surrounds the plasma arc and is used for constriction of the plasma arc width and also cools the torch. The shield gas creates the cutting environment and the edge quality.
Such cutting processes have certain limitations when cutting high quality bolt holes. For example, if a plasma arc torch is in a perpendicular position throughout the circumference of the hole, a considerable “tapered” or “beveled” cut will occur. The diameter of the bolt hole on the top side of the workpiece will be larger than the diameter of the bolt hole on the bottom side of the workpiece thus not achieving absolute cylindricity. Cylindricity in this instance is defined as a, three-dimensional geometric tolerance that controls how much a feature can deviate from a perfect extruding cylinder through the workpiece. In these types of instances, secondary processes such as burrowing or drilling and required to enlarge the diameter of the bolt hole on the bottom side of the workpiece. Secondary processes are time consuming and unconventional suggesting that a more efficient method is necessary for cutting high quality bolt holes. Furthermore, prior processes left irregularities in the interior of the bore hole, requiring further operations to reduce them.